Requirements of the Untied States Department of Homeland Security include a need for devices capable of sensitive detection of gamma rays originating from hidden radioactive material (e.g. in accordance with standards of the American National Standards Institute (ANSI) such as ANSI N42.32). As well, many steel plants and scrap yards are concerned about the potentially dangerous melting of so-called orphane sources which might be included in the in or outbound scrap material. Even landfills and waste incineration plants equip their gates and personal with monitors for the detection of such radioactivity. Commercially available high sensitivity portable or mobile gamma radiation meters can be deployed to detect very small amounts of radioactivity.
Some conventional radiation detection instruments simply display the number of detected gamma rays sensed (i.e., counted) by the device, while other conventional radiation detection devices are capable of measuring and displaying the dose rate of the gamma radiation field detected by the device. Operators of such devices can set alarm thresholds on absolute numbers of the detected particles per time unit or on the measured dose rate, depending upon the device used. Some radiation detection systems are configured to generate an alarm when the respective count or dose rate of gamma radiation exceeds a predetermined threshold related to background level.
Prior to actual use, and preferably on a regular basis during their useful life, radiation detection devices typically should be calibrated against a known standard. Calibration can require at least periodic exposure of a radiation detection device to a radioactive source exhibiting a similar spectra of energy as those radioactive sources of concern.
Conventional manufacturing of radioactive sources for calibration of radiation detectors (e.g., so-called check sources) typically requires access to a reactor or an accelerator to produce the radioactive material. The man-made isotopes used as check sources typically exhibit a half-life between a few minutes and several years; those with short half-lives require frequent replacement.
For many reasons, radioactive sources often need to be very strong (e.g., emitting a high amount of radiation). Accordingly, such sources require special handling during use as well as storage. Government authorities have established rules and regulations in order to protect workers and the public from any possible danger from these sources. Unfortunately, this can hamper the possession and usage of even small amounts of such radioactive material.
Commercially available high sensitivity, stationary, portable or mobile gamma radiation meters can easily detect very small increases in the strength of a gamma radiation field. However, a problem arises when such devices are deployed to users who normally do not handle radioactive materials and who therefore do not own corresponding check sources to properly test the performance of the detectors.
As an alternative to the use of man-made radioactive material, certain naturally occurring radioactive materials have been used to verify the performance of radiation detection devices. However, the only natural materials known to be used as check sources today are K-40, isotopes of the Th-232 decay chain, and isotopes of the U-238 decay chain.
Material such as incandescent mantles (Thorium), old watches (Radium) and fertilizer (Potassium K-40) can emit suitable levels of radioactivity for testing purposes. The elements Thorium and Uranium exhibit multiple spectral energies ranging up to 3 Megaelectron Volts; K-40 produces a single spectral line at about 1.5 Megaelectron Volts. However, these isotopes are not well suited to test portal monitors or pocket size scintillation detectors because their average gamma energy is significantly higher than the typical gamma energies of those isotopes of concern.